The invention relates to oxygen sensors and particularly to such sensors which utilize solid electrolytes which must generally operate at temperatures of at least about 800.degree. F. In many situations, particularly the automotive field, the sensor's environment can maintain a sufficiently high temperature. Where it cannot, such as in many industrial applications, a separate heater must be provided. Even in some installations where there is sufficient heat, it is often desirable to maintain the sensor at some constant temperature somewhat above the maximum temperature of the process. With this latter arrangement the cell output is directly related to the familiar Nernst equation and can represent percent oxygen without the necessity of providing complex circuitry to account for variable temperatures.
Typically, oxygen sensors are constructed from ceramic materials in a tubular shape. An example of such a sensor for industrial use is Shum et al U.S. Pat. No. 4,119,513. Heaters, if provided, are generally constructed as separate items surrounding the sensor. This method interposes a thermal barrier between the heat source and sensor which can require heater temperatures considerably above the desired sensor temperature. These high temperatures require costly materials of construction for long life. If attempts are made to bond a separate heater to the sensor, then problems in materials arise. The heater material, adhesives, and sensor materials must be matched so as to prevent thermal stresses from arising, particularly during fast heat-up cycles. In addition many of the available high temperature adhesives require curing temperatures in excess of the allowable limits of the sensor and therefore are unusable. Some examples of gas sensor patents which include heating in one way or another are: U.S. Pat. Nos. 3,442,773; 3,597,345; 3,616,408; 3,871,981; 3,911,386; 3,915,828; 3,936,794 and 4,004,452.